Radioactivity

With radioactivity making news headlines, we look at what it is and what the risks are.

Radioactive decay occurs in unstable atomic nuclei – that is, ones that don’t have enough binding energy to hold the nucleus together due to an excess of either protons or neutrons.

It comes in three main types – named alpha, beta and gamma for the first three letters of the Greek alphabet.

Alpha decayAn alpha particle is identical to a helium nucleus, being made up of two protons and two neutrons bound together.

It initially escapes from the nucleus of its parent atom, invariably one of the heaviest elements, by quantum mechanical processes and is repelled further from it by electromagnetism, as both the alpha particle and the nucleus are positively charged.

The process changes the original atom from which the alpha particle is emitted into a different element.

Its mass number decreases by four and its atomic number by two. For example, uranium-238 will decay to thorium-234.

Sometimes one of these daughter nuclides will also be radioactive, usually decaying further by one of the other processes described below.

Beta decayBeta decay itself comes in two kinds: β+ and β-.

β- emission occurs by the transformation of one of the nucleus’s neutrons into a proton, an electron and an antineutrino. Byproducts of fission from nuclear reactors often undergo β- decay as they are likely to have an excess of neutrons.

β+ decays is a similar process, but involves a proton changing into a neutron, a positron and a neutrino.

Gamma decayAfter a nucleus undergoes alpha or beta decay, it is often left in an excited state with excess energy.

Just as an electron can move to a lower energy state by emitting a photon somewhere in the ultraviolet to infrared range, an atomic nucleus loses energy by emitting a gamma ray.

Gamma radiation is the most penetrating of the three, and will travel through several centimetres of lead.

Beta particles will be absorbed by a few millimetres of aluminium, while alpha particles will be stopped in their tracks be a few centimetres of air, or a sheet of paper – although this type of radiation does the most damage to materials it hits.

Half-lives and probabilityRadioactive decay is determined by quantum mechanics – which is inherently probabilistic.

So it’s impossible to work out when any particular atom will decay, but we can make predictions based on the statistical behaviour of large numbers of atoms.

The half-life of a radioactive isotope is the time after which, on average, half of the original material will have decayed. After two half-lives, half of that will have decayed again and a quarter of the original material will remain, and so on.

Uranium and plutonium are only weakly radioactive but have very long half-lives – in the case of uranium-238, around four billion years, roughly the same as the current age of the Earth, or the estimated remaining lifetime of the Sun. So half of the uranium-238 around now will still be here when the Sun dies.

Iodine-131 has a half-life of eight days, so, once fission has stopped, less than 1% of iodine-131 produced in a nuclear reactor will remain after about eight weeks. Other radioisotopes of iodine are even shorter-lived.

Caesium-137, however, sticks around for longer. It has a half-life of around 30 years, and, because of this and because it decays via the more hazardous beta process, is thought to be the greatest health risk if leaked into the environment.

Although some radioactive materials are produced artificially, many occur naturally and result in there being a certain amount of radiation in our environment all the time – the “background radiation”.

In the backgroundThere is a natural level of radiation all around us, which comes from several sources.

Some gamma radiation comes from space as cosmic rays. Other radiation comes from sources in the atmosphere, such as radon gas and some of its decay products.

There are also natural radioactive materials in the ground – and as well as the obvious elements such as uranium there are also radioactive isotopes of common substances such as potassium and carbon.

To understand how much background radiation is around, it helps to distinguish between effects on normal matter and on the human body.

The amount of radiation absorbed by non-biological matter is measured in grays, a unit equivalent to a joule of energy per kilogram of mass. For biological tissue, a dose equivalent is measured in sieverts (Sv) depending on the type of radiation involved and how much damage that radiation does to the particular cells affected.

The dose equivalent in sieverts is the dose in grays multiplied by some “quality factor” for the type of tissue irradiated and for the type of radiation – for electrons or gamma-rays, 1; for alpha particles such as those given off by the radioactive decay of uranium, 20.

The average amount of radiation received from background sources in the UK is around 2–2.5 mSv per year. Because of the preponderance of granite, which contains higher than average levels of uranium, in areas such as Cornwall or Aberdeenshire it can be twice this level – not high enough to cause any concern, but high enough that nuclear facilities can’t be built there as the background level already exceeds the maximum allowed radiation limit. In some parts of the world, such as northern Iran, the background radiation is as high as 50 mSv per year.

There are a variety of other natural and routine artificial causes of low doses of radiation.

A dental x-ray will give you a dose of under 1 mSv; a full-body CT scan, 10 mSv.

As fewer cosmic rays are stopped by the atmosphere the higher you go, the crew of a passenger jet flying between the US and Japan once a week for a year would receive an additional a dose of around 9 mSv.

Under normal conditions, the dose limit for workers in the nuclear industry is 50 mSv per year.

The effects on human healthThere are two main health effects caused by radiation, which act over the short- and long-term and also at shorter and greater distances.

Radiation causes health problems by killing cells in the body, and the amount and type of damage done depends on the dose of radiation received and the time over which the dose is spread out.

The dose limits for emergency workers in the event of a nuclear accident are 100 mSv if protecting property or 250 mSv in a life-saving operation.

Between that upper limit and 1 Sv received within a single day, exposure is likely to cause some symptoms of radiation poisoning, such as nausea and damage to organs including bone marrow and the lymph nodes. Up to 3 Sv these same effects are more serious with a likelihood of acquiring infections due to a reduced number of white blood cells in the body – with treatment, survival is probable but not guaranteed.

Larger doses will, in addition to those symptoms above, cause haemorrhaging, sterility and skin to peel off; an untreated dose of more than 3.5 Sv will be fatal, and death is expected even with treatment for doses of more than 6 Sv.

The radiation level decreases with the square of the distance from its source, so someone twice as far away from an external source will receive a quarter of the radiation.

Receiving a high dose in a shorter time usually causes more acute damage, as greater doses kill more cells, while the body can have had time to repair some damage with more time having elapsed between doses.

However radioactive material that is spread to a wider area can cause longer-term health effects via prolonged exposure, particularly if they enter the food chain or are inhaled or ingested directly.

Taking radioactive materials into the body also presents the greatest danger from atoms that undergo alpha-decay, as alpha particles are not very penetrative and are easily absorbed by a few centimetres of air. It was alpha-emitting polonium-210 that was used to murder Alexander Litvinenko in 2006.

Radioactive isotopes of iodine, which undergo beta-decay, can build up in the thyroid gland and can cause thyroid cancer. Attempts to prevent this involve distributing pills that include nonradioactive iodine-127 and which flood the thyroid, preventing uptake of radioactive iodine.

For one-off doses, such as those from medical scans, the risk of later developing cancer is estimated at around 1 in 20 000 per mSv received.

Absorbing an accumulated dose of 1 Sv over a longer period of time is estimated to eventually cause cancer in 5% of people.

However there is disagreement over whether very small doses comparable to the level of background radiation actually contribute to health effects.